misc: Adapt consistent import style formatting

Historically the Cloud Hypervisor coding style has been to ensure that
all imports are ordered and placed in a single group. Unfortunately
cargo fmt has no support for ensuring that all imports are in a single
group so if whitespace lines were added as part of the import statements
then they would only be odered correctly in the group.

By adopting "group_imports="StdExternalCrate" we can enforce a style
where imports are placed in at most three groups for std, external
crates and the crate itself. Choosing a style enforceable by the tooling
reduces the reviewer burden.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

vm-virtio/src/lib.rs

@@ -12,6 +12,7 @@
 
 use std::fmt::{self, Debug};
 use std::sync::Arc;
+
 use virtio_queue::{Queue, QueueT};
 use vm_memory::GuestAddress;
 

vm-virtio/src/queue.rs

@@ -11,6 +11,7 @@
 pub mod testing {
     use std::marker::PhantomData;
     use std::mem;
+
     use virtio_queue::{Queue, QueueT, VirtqUsedElem};
     use vm_memory::{bitmap::AtomicBitmap, Address, Bytes, GuestAddress, GuestUsize};